Recently, as a power supply for an electronic device, such as a notebook computer and a mobile phone, attention has been focused on a fuel cell capable of supplying power continuously for a long period. Various types of fuel cell have been developed, and as a power supply for a mobile electronic device, such as a notebook computer, that demands a reduction in size, a fuel cell of a type that directly supplies power without reforming fuel by a reformer, for example, known as a direct methanol fuel cell (hereinafter, abbreviated as DMFC) shows great promise.
The DMFC includes a fuel circulation type that collects an unused fuel cell from supplied methanol for reuse, and a fuel non-circulation type that does not reuse unused methanol. The fuel circulation type can readily obtain a stable generated output by stabilizing the operating point. It has, however, a drawback that the need for a collecting mechanism (circulation pump or the like) to collect unused methanol complicates the configuration and the device is increased in size. Meanwhile, the fuel non-circulation type can achieve a size reduction of the device because it does not need the collecting mechanism. However, it is not preferable to release a large volume of methanol, which is poisonous. In addition, in terms of enhancing generation efficiency, it is crucial to use up supplied methanol almost completely, that is, to allow supplied methanol to burn completely.
FIG. 11 is a graph showing the current-to-voltage characteristic, the current-to-power characteristic, and the release ratio characteristic for a quantity of supplied methanol (fuel) in the DMFC. Referring to FIG. 11, the ordinate is used for an output voltage (V), output power (W), and a release ratio (%) of the DMFC, and the abscissa is used for an output current (A) of the DMFC. C11 through C13 are the current-to-voltage characteristic curves when quantities of supplied fuel are 0.1 cc/min, 0.2 cc/min, and 0.3 cc/min, respectively. C21 through C23 are the current-to-power characteristic curves when quantities of supplied fuel are 0.1 cc/min, 0.2 cc/min, and 0.3 cc/min, respectively. C31 shows a relation of an output current and a release ratio when a quantity of supplied fuel is 0.3 cc/min. The release ratio is defined as a ratio of released fuel with respect to supplied fuel expressed in percentage.
As is shown in FIG. 11, it is understood that higher output power can be obtained as a quantity of supplied fuel is increased. Also, as are indicated by C11 through C13, it is understood that an output voltage decreases as an output current increases. Further, as is indicated by C31, it is understood that a release ratio decreases as an output current increases.
In the following, an example will be described using a case where a quantity of supplied fuel is 0.3 cc/min. As is indicated by C13, it is understood that the voltage decreases slowly until the output current reaches A3, whereas the voltage decreases abruptly when the output current exceeds A3. Meanwhile, as is indicated by C31, it is understood that the supplied fuel is used up almost completely by the time the output current reaches A3. It is therefore preferable to set the operating point of the fuel cell to a point at which an output current is larger than A3 in terms of allowing the fuel to burn completely. This, however, increases an output current only slightly, and causes the output voltage to decrease abruptly, which makes it impossible to supply a stable output voltage to the load device.
For the fuel non-circulation type, it is therefore required not only to set the operating point in proximity to the power maximum point P, but also to severely control the operating point not to vary. Patent Document 1 discloses a fuel cell voltage generator comprising a DC-DC converter connected to the output side of the fuel cell, a rechargeable battery connected to the output side of the DC-DC converter, and a switch controller that supplies a PWM signal to the DC-DC converter, in which the switch controller calculates a duty ratio of the PWM signal on the basis of a difference between an output voltage of the fuel cell and the reference value.
Also, Patent Document 2 discloses a power supply comprising a fuel cell, a DC converter, a rechargeable battery, and a micro processor that controls the DC converter, in which the maximum value of a current flowing into the DC converter is varied for the voltage of the fuel cell to fall within a specific range including the maximum power.    Patent Document 1: U.S. Pat. No. 6,590,370 B1    Patent Document 2: U.S. Pat. No. 5,714,874